The human eye in its simplest terms functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a crystalline lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and the lens.
When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (IOL).
Cataractous lenses may be removed by a surgical technique called phacoemulsification. During this procedure, an opening is made in the anterior capsule and a thin phacoemulsification cutting tip is inserted into the diseased lens and vibrated ultrasonically. The vibrating cutting tip liquifies or emulsifies the lens so that the lens may be aspirated out of the eye. The diseased lens, once removed, is replaced by an artificial lens.
In the natural lens, bifocality of distance and near vision is provided by a mechanism known as accommodation. The natural lens, early in life, is soft and contained within the capsular bag. The bag is suspended from the ciliary muscle by the zonules. Relaxation of the ciliary muscle tightens the zonules, and stretches the capsular bag. As a result, the natural lens tends to flatten. Tightening of the ciliary muscle relaxes the tension on the zonules, allowing the capsular bag and the natural lens to assume a more rounded shape. In this way, the natural lens can focus alternatively on near and far objects.
As the lens ages, it becomes harder and is less able to change shape in reaction to the tightening of the ciliary muscle. This makes it harder for the lens to focus on near objects, a medical condition known as presbyopia. Presbyopia affects nearly all adults over the age of 45 or 50.
Prior art accommodative two lens systems using a movable “zoom” lens have inherently limited movement. The maximum sensitivity or movement magnification a (a unitless ratio) is defined as the axial movement of the lens per unit zonule movement and is derived by the following equation:a=−B/A where B is the projected distance of the zonule length which is in the order of 1.0 to 2.0 mm; and A is the axial distance between the middle plane of the dual lenses and the anterior surface of the anterior lens where the zonules terminate.
U.S. Patent Application Pub. No. US2007/0050024, the entire contents of which are incorporated herein by reference, discloses the use of a cam mechanism to increase the range of relative movement between the elements of a two-optic system. However, even with a cam element or other mechanism for increasing the range of movement in dual optic systems, it is difficult to obtain an accommodative amplitude that would restore the normal accommodation of a healthy eye, e.g., a power shift on the order of 4 diopters, due to the refractive limitations of conventional lens materials and the limited space available within the capsule. Consequently, patients can have refractive errors after the implantation of the IOL and still need additional spectacles corrections that are not desired.
Accordingly, there exists a need for better solutions to the problem of accommodation in IOLs. In particular, a single optic accommodative lens that could mimic the natural (young) lens and provide greater accommodative amplitude would satisfy a long-felt need in the field.